1. Field of the Invention
The present invention relates to aqueous coating compositions containing urethane (meth)acrylates and binders containing isocyanate-reactive groups and to their use for coating various substrates, especially metal substrates.
2. Description of the Prior Art
The hardening of coating compositions containing meth)acryloyl groups by UV-initiated polymerization which proceeds by a free radical mechanism is a known and industrially established procedure. It is one of the most rapid methods of hardening in coating technology. However, UV-hardening lacquers have certain disadvantages, such as their high volume shrinkage during hardening, which can result in adhesion problems. The high crosslink density which is inherent in the system results in brittleness, lack of flexibility and a lack of impact strength. Also, the hardening of coating compositions containing UV-hardening (meth)acryloyl groups is dependent on the availability of a sufficient dosage of radiation of the appropriate wavelength. Poorly exposed regions suffer from reduced hardening, which results in appreciable losses in surface resistance, and unexposed regions do not harden at all.
Coatings are known which are prepared from aqueous binder compositions containing polyisocyanates and compound with isocyanate-reactive groups, preferably polyols as described, e.g., in EP-A 358,979.
Due to the wide choice of reactants, the technical properties of these coatings, such as adhesion, flexibility, impact strength and resistance to weathering, can be varied over a wide range. However, compared with radiation-hardening systems, the hardening of aqueous two-component polyurethane compositions requires a long time, even at elevated temperatures. Another difference is that UV light is not necessary for curing.
Combinations of both types of hardening, i.e., so-called dual cure systems hardening by UV radiation and hardening by polyurethane formation, are also known for solvent-based lacquers. For example, U.S. Pat. No. 4,342,793 describes hardenable resin compositions which contain a radiation-hardenable reactive thinner (low molecular weight esters of acrylic acid), a saturated polyol and a polyisocyanate. Hardening is carried out by irradiation in order to polymerize the reactive thinner, which is followed by thermal hardening to form a polyurethane from the polyol and the polyisocyanate.
One disadvantage of these compositions is that the solvent content and thus the level of emissions is relatively high. In addition, if the irradiation conditions are unfavorable, any reactive thinner which is not chemically incorporated can remain behind in the hardened lacquer film. This can result in problems, e.g., if the lacquer film contacts skin. Also, there may be a negative effect on the properties of the lacquer film, such as its hardness and physical and chemical resistance.
An object of the present invention is to provide a coating composition which contains an aqueous binder and in which lacquered parts may be rapidly handled due to UV hardening. It is an additional object of the present invention to be able to vary the properties depending upon the particular application by the choice of crosslinking constituents, preferably polyols containing active hydrogen the present invention to achieve satisfactory hardening even in unexposed or poorly exposed regions.
Surprisingly, these objects may be attained with the coating compositions of the present invention, which contain urethane (meth)acrylates having (meth)acryloyl groups and optionally free NCO groups, and also aqueous polyols and optionally polyisocyanates. In these compositions UV-hardening can be carried out to produce coatings that can be rapidly handled. Subsequent crosslinking by the reaction of NCO then proceeds to completion and results in coatings which exhibit high chemical and mechanical resistance. In areas of poor exposure if there is a certain residual density of double bonds, crosslinking is ensured via the reaction of NCO and thus a minimum required level of resistance is ensured.
This is surprising since it had been assumed that the two reaction processes would impede each other, i.e., the rapid UV-initiated polymerization would xe2x80x9cfreeze-inxe2x80x9d the slower reaction of NCO, such that the reaction of NCO groups with compounds containing isocyanate-reactive groups would not proceed to completion.
The present invention relates to an aqueous coating composition containing
a) a urethane (meth)acrylate having (meth)acryloyl groups and optionally free isocyanate groups,
b) optionally a further polyisocyanate,
c) a UV initiator which initiates polymerization by a free radical mechanism, and
d) one or more aqueous binders containing isocyanate-reactive groups.
The present invention also relates to the use of the coating compositions for coating various substrates, especially metal substrates.